The production of homo- and copolymers of vinyl chloride by polymerization in an aqueous emulsion is well known, e.g., as described in the monograph by Kainer, "Polyvinylchlorid und Vinylchlorid-Mischpolymerisate" (Polyvinyl Chloride and Copolymers of Vinyl Chloride), Springer publishers, Berlin/Heidelberg/New York, 1965, pp. 34 et seq.
During emulsion polymerization of vinyl chloride, polymerization generally is not continued until a quantitative conversion of the monomers has been attained. In many cases, the polymerization is also deliberately interrupted at a low conversion. In all cases, considerable amounts of residual mnomers remain in the dispersions.
The removal of a large part of unreacted monomers is conventionally effected by expansion and/or evacuation customarily within the polymerization reactor; the vinyl chloride, removed by suction, is recycled to a gasometer. In spite of these measures for the recovery of vinyl chloride, the aqueous dispersion still contains about 1-2% by weight of residual monomer, based on the polymer. During the spraydrying of dispersions obtained from emulsion polymerization, more than 95% by weight of the monomer content escapes, together with the dryer exhaust air, into the atmosphere. In the spray-drying of polyvinyl chloride, only physiologically acceptable residual monomer contents on the order of 1-20 p.p.m. remain in the polymer composition.
However, in conjunction with constantly increasing efforts to reduce environmental pollution, the problem has arisen of how to reduce the heretofore considerable emissions of vinyl chloride in the dryer exhaust air. For this purpose, it is necessary to extensively free the polyvinyl chloride dispersions of monomeric impurities prior to the drying step.
It has been known to free synthetic resin dispersions of readily volatile monomers by storage in large expansion tanks, e.g., as taught in German Pat. No. 1,248,943, column 2, lines 4-6. However, to be effective on a technical scale, this mode of operation entails an enormous expenditure in capital equipment.
Another prior art process passes polymer dispersions such as polyvinyl chloride through trickling adsorbers countercurrently to rising inert gases to thereby free these dispersions from readily volatile compounds, e.g., see DOS (German Unexamined Laid-Open Application) No. 2,162,860. This mode of operation likewise requires excessive expenditures for capital equipment.
Steam distillation of polymer dispersions for the removal of readily volatile components has also been known, e.g., see German Pat. No. 1,248,943, column 2, lines 24-28. However, this procedure cannot be readily utilized for polymer dispersions obtained during emulsion polymerization, since these dispersions have such a low surface tension due to their emulsifier content that strong foam formation occurs during such a steam distillation.
In one conventional, discontinuously operating method described in German Pat. No. 1,248,943, the foam produced during steam distillation of dispersions having a surface tension of below 40 dyne/cm. is broken up by subjecting the thus-formed steam-dispersion foam mixture to a rapid pressure drop at flow speeds of more than 100 m./sec. The thus-broken up foam is returned from the separator to the steam distillation for recycling.
As is expressly emphasized at column 5, lines 19-22, of German Pat. No. 1,248,943, such a process can be operated continuously only with connection of several units in series as a cascade. Naturally, such a series connection of several discontinuously operating units requires great expenditures for the apparatus. Additionally, series connection of several units of the above-described type results in a great mechanical load on those dispersions which have a high solids content and a low emulsifier content, which are almost exclusively produced in modern large-scale technical processes. In a highly undesirable manner, deposits are formed from the comparatively low stability dispersions, especially in the pipelines between the degasification tanks.